Material sparking method and apparatus



June 14, 1966 G. D. PERKINS ETAL.

MATERIAL SPARKING METHOD AND APPARATUS Filed Jan. 23, 1963 United States Patent O Filed Jan. 23, 1963, Ser. No. 253,374 4 Claims. (Cl. Z50-41.9)

The subject invention relates to a method and apparatus for producing ions from a sample of material and, more particularly, a method and apparatus for analyzing material in a mass resolving system.

The expression mass resolving system as used herein is intended to refer to mass spectrometers, mass spectrographs and similar apparatus and systems.

Mass revolving systems of the type referred -to above are well known in the art and are in wide use for analyzing various materials. Many of these systems employ a spark ion source in which solid or liquid samples are vaporized and ionized by means `of an electric spark.

In sparking the sample it is essental that the temperature rise at the sample be kept to a minimum in order to prevent fractionation of the sample which would give higher comparative outputs for the lower melting point materials contained in the sample and thus introduce analysis errors. In addition, the ion current through the trajectory of the material beam of the mass revolving system should be kept below the level at which the space charge in the beam will broaden such beam beyond the limit of useful resolution. To meet these requirements, the prior art systems have employed controlled power sources that supply the voltage for the electric spark intermittently in short time periods, such as in the form of an alternating-current high-frequency potential or unipolarity pulse potentials.

However, these controlled power sources include elaborate and complex electronic equipment for the control of the spark voltage and duration. In addition, the frequent interruption of the sparking voltage limited the ion yield obtainable from the spark ion source.

The subject invention overcomes these disadvantages and provides a method and apparatus for producing ions from a sample of material wi-thout the need of the complex prior art control equipment, without subjecting the sample to undue temperature rises and without unduly limiting the ion yield obtainable from the sample of material.

The method of the invention broadly comprises the steps of preparing two spaced electrodes of the sample of material to be analyzed or two spaced electrodes having such sample applied thereto. According to the invention, an electrical impedance having two terminals is prepared and one terminal thereof is connected -to one of the electrodes. An electric current is then applied to the other terminal of the impedance and the other of the two electrodes. The voltage of such current is suicient to produce .an

. electric spark between the two electrodes so as to resolve the sample to its constituents in ionized form. The electrodes are ordinarily maintained in an evacuated space, such as the vacuum space normally present in a mass resolving system.

Employing the method of the subject invention, we have found that undue temperature increments at the sample can be avoided, since the impedance employed will limit the spark current to a desired value without the assistance of complex control circuitry. At the same time, the ion yield of the sample reaches high values, since the sparking voltage need not be intermittently interrupted at a high rate to prevent overheating of the sample. Moreover, the impedance employed limits the ion current through the above-mentioned beam trajectory so that a precise resolution is maintained.

The apparatus o f the invention broadly comprises -a pair of spaced spark gap electrodes formed of the sample of material to be analyzed or having such sample applied thereto, and means for evacuating the space occupied by the electrodes. According to the invention, vthe apparatus .includes an electrical impedance having a first terminal connected to one of the electrodes and having a second terminal, and means for applying a high voltage to the other electrode and the other impedance terminal to produce a material resolving spark between the electrodes.

Advantageously, the high voltage applied to the electrodes through the above-mentioned impedance may be a direct-current voltage or a potential having a predominant direct-current component to obtain optimal ion yield from the sample.

In many cases, the high voltage employed in mass resolving systems for biasing accelerating and similar electrodes will also be found suitable for generating the spark between the above-mentioned sparking electrodes. In this manner, a separate sparking voltage source can be dispensed with.

If the sparking voltage is a direct-current potential, the impedance should be a direct-current conducting impedance. Thus, a resistor may be used as the above-mentioned impedance. The resistance value of such resistor may vary within relative wide limits depending, for instance, on the type or composition of material sparked and the value of the sparking voltage employed.

Thus, we have found that a lower resistance value can be used if the material has no low melting point components, while a higher resistance value should be employed when low melting point components are present in the material sample. By way of example, we have suc- -cessfully experimented with various resistors having resistance values in the range of from 5 megohms to 100 megohms and sparking potentials having a vol-tage of from 15 kv. to 50 kv. We have also found that the accuracy of the test results obtained depends to a certain extent on the value of the resistor employed with a given voltage, even where relatively pure materials are analyzed. Thus, we have found that good results are obtained with a resistance value of as high as megohms and a sparking voltage of about 15 kv. when carbon is analyzed, while resistance values as low as 5 megohms at the same voltage had to be employed in .the analysis of silicon, for instance.

The resistance values and voltages given above may, of course, be subject to wide variation depending on the parameters mentioned above.

The invention and its objects will become more readily apparent from the following detailed description of preferred embodiments, illustrated by way of example, in the accompanying drawings, in which:

FIG. l diagrammatically shows a mass spectrometer apparatus incorporating the subject invention;

FIG. 2 shows a modified detail of the apparatus of FIG. 1; and

FIG. 3 shows another modification of the detail illustrated in FIG. 2.

` The mass spectrometer diagrammatically illustrated in FIG. 1 will be recognized as being of the Mattauch type. This type of spectrometer is well known in the art and has, for example, been described in the Zeitschrift fur Physik, 98, page 786, 1934, and also in U.'S. Patent No` 2,851,608, Mass Spectrometer, issued September 9, 1958, to Charles F. Robinson, and assigned to the subject assignee.

In brief, the illustrated spectrometer comprises a sparking source 10, an accelerating electrode 11, further apertured electrodes 12 and 13', a pair of curved electrostatic deector electrodes 14 and 15, an additional apertured electrode 16, a magnetic structure providing a transverse field and comprising a pair of magnetic poles of which one is shown at 17, and ion sensing means, such as a photographic plate 18. The accelerating electrode 11 is electrically biased by means of an accelerating voltage supply 20 connected between electrode 11 and ground. The deflecting electrodes 14 and 15 are properly biased by means of a deecting voltage supply 21 which is connected to electrodes 14 and 15 and a resistor 22 having a grounded center tap. The parts so far described with reference to FIG. 1 are well known in the art so that further description thereof is unnecessary.

The ions released at source 10 travel to the magnetic structure comprising pole 17 in the conventional manner substantially along a trajectory schematically indicated by dotted line 25. In the magnetic structure, the ions are separated to move along individual trajectories, such as the ones indicated schematically by dotted lines 26, 27, and 28, so that they impinge on different points on plate 18. The principle so far described with reference to FIG. 1 is also well known in the art.

The sparking source 10 comprises a pair of electrodes 30 and 31 which are formed of the material to be sparked and analyzed. A sparking voltage supply 32, which preferably is a high voltage direct-current supply, is connected between electrode 31 and ground. A resistor 34 is connected between electrode 3G and ground. As has been mentioned above, the resistance of resistor 34 may vary in the range of from 5 megohms to 100 megohms or may have a value in the vicinity of such range, depending on the voltage of supply 32 and the sparking characteristics of the material being analyzed.

During operation of the sparking source 10, the electrodes 30 and 31 occupy the evacuated space in which electrodes 11 to 16, the magnet structure including pole 17 and the plate 18 are located, as is well known. The supply 32 provides a continuous sparking voltage sufficient to generate an electric spark between electrodes 30 and 31, and resistor 34 limits the sparking current in the manner aforesaid, whereby the material of electrodes 30 and 31 is sparked without being subjected to an undue temperature rise. The sparked material will move from electrodes 30 and 31 along trajectory 25 in a manner well known per se.

FIG. 2 shows a sparking source 10' which incorporates a modification of the sparking source shown in FIG. 1. According to FIG. 2, the accelerating voltage supply 20 is not only used to bias accelerating electrode 11 but also to supply the sparking voltage to electrodes 30 and 31. To this end, electrode 31 is connected to supply 20 as shown, while electrode 30 is connected to resistor 34 which, in turn, is connected to ground. In this manner, the benefits of the invention can be obtained without employing a separate sparking voltage supply of the type shown in FIG. 1 at 32. With the embodiment shown in FIG. 2, it is, of course, necessary that the voltage supply 20 provide a potential sufficient to generate the desired spark between electrodes 30 and 31. In practical experiments we have employed the standard l5 kv. accelerating voltage supply of an existing mass spectrometer which provided excellent results with resistors 34 of 5 megohms and more.

If desired, the electrodes 30 and 31 need not necessarily be formed of the material to be analyzed. Instead, these electrodes may be formed of another material, such as graphite, and a sample of the material to be -analyzed may be applied to one of the electrodes. An arrangement of this type is illustrated in FIG. 3 which shows a pair of electrodes 40 and 41 that are formed of an electroconductive material other than the material to be analyzed. A sample 42 of the material to be analyzed is applied to electrode 40. During sparking, the electric spark generated between electrodes 40 and 41 will cause the material of sample 42 to be Vaporized and ionized and to travel along trajectory in the aforesaid manner. Any material so removed from electrodes 40 and 41 will be recognized as emanating from these electrodes, since they are made of a material other than that of sample 42. In this manner, it is possible to analyze materials that are not electric conductors and that could, therefore, not be employed to form the sparking electrodes. This technique may also be employed to analyze liquids rather than solids. In this case, the electrodes 40 and 41 are made of a porous material, such as porous graphite, and are immersed in the liquid to be analyzed prior to injection in the spectrometer apparatus. It will, of course, be understood that the electrode arrangement shown in FIG. 3 could also be employed in the embodiments shown in FIGS. 1 and 2, while the apparatus shown in FIG. 3 could also be provided with electrodes of the type of electrodes 30 and 31 of FIGS. 1 and 2.

In FIG. 3, the accelerating voltage supply 20 is connected between ground and one of the sparking electrodes in a manner as shown in FIG. 2. However, the resistor 34 is not connected directly to ground as in FIG. 2, but is connected to a supplementary voltage supply 50 which is interposed between resistor 34 and ground. This supplementary voltage supply serves the purpose of boosting the voltage of supply 20 to obtain a sparking voltage that is h'igher than the potential provided by supply 20. In this manner, the sparking voltage may be made higher than the accelerating voltage without the need of a separate voltage supply that is capable of providing the entire sparking Voltage. Thus, if the accelerating voltage supply provides a potential of 15 kv. and it is desired to have a sparking voltage of 2O kv., a supplementary voltage supply 50 having an output voltage of 5 kv. is interposed between resistor 34 and ground as shown. In this manner, a 5 kv. supply can be employed instead of a more expensive, separate 20 kv. supply. The supplementary voltage supply 50 preferably is a direct current voltage supply. However, if it is desired that the sparking voltage have an alternating current component, an alternating current supply or a voltage pulse source may be employed as supplementary voltage supply 50. In this manner, the temperature rise of the material being analyzed is not only controlled by the resistor 34, but also by the amplitude and frequency of the supplementary alternating current voltage or the amplitude and rate of succession of the supplementary pulse voltage.

While it is believed that the best ion yield will be obtained by employing a direct current voltage source as the sparking voltage supply 32 shown in FIG. 1, it should be understood that this supply 32 may also be a source of alternating current or a source supplying a direct current having an alternating component.

It will also be understood that the sparking sources shown in FIGS. 1, 2 and 3 may be employed in connection with mass resolving systems other than that shown refer also to two or more resistors or impedance elements.

In all these cases, the method and apparatus of the invention will provide sparking sources that enable optimum ion yield without the assistance of complex control circuitry and without subjecting the material being analyzed to overheating and the mass trajectory to excessive transverse ion current.

Various modications within the scope of the invention will be apparent to those skilled in the art.

We claim:

1. In a mass resolving device including an ion accelerating electrode, -means connected to the accelerating electrode for impressing upon 4the accelerating electrode a predetermined high-voltage direct-current potential different from ground potential, and lirst and second spaced electrode, the spark-gap electrodes incorporating a sample spark-gap electrodes disposed adjacent the accelerating of non-gaseous material to be ionized, apparatus for producing ions of the sample accurately representative of the quantitative composition of the sample comprising circuit means coupled to the spark-gap electrodes for impressing a selected substantially constant direct-current potential across the spark-gap electrodes suilicient to produce an arc across the space between the spark-gap electrodes and for causing the rst spark-gap electrode to have a potential substantially equal to said predetermined potential, the circuit means including impedance means for limiting the current which ows between the spark-gap electrodes during the existence of an arc across said spark to a value insutlcient to heat said sample to a temperature at which the sample ionizes selectively in relation to the volatility of the constituents thereof and for assuring that the second spark-gap electrode has a potential different from ground potential during the existence of said arc.

2. Apparatus according to claim 1 wherein thecircuit means includes a high-voltage, direct-current power supply having a first terminal connected to ground and a second terminal connected both to the accelerating electrode and the lirst spark-gap electrode.

3. Apparatus according to claim 2 wherein the impedance means comprises a conductive electrical impedance device having a first terminal connected to the second spark-gap electrode and a second terminal connected to ground.

4. Apparatus according to claim 2 wherein the circuit means includes a conductive electrical impedance device having `a first terminal connected'to the second sparkgap electrode and a second terminal, and a second directcurrent power supply having a rst terminal connected to the second `terminal of the impedance device and a second terminal connected to ground for driving the second terminal of the impedance device to a potential below ground whereby said selected potential is greater than said predetermined potential.

References Cited by the Examiner UNITED STATES PATENTS 2,285,322 6/1942 Anderson 315-237 2,456,116 12/1948 Enns 315-237 2,480,681 8/ 1949 Steifel 315-237 2,541,877 2/1951 Machler 315-237 2,735,330 2/1956 Polster 315-237 2,851,608 9/1958 Robinson Z50-41.9

RALPH G. NILSON, Primary Examiner.

H, S. MILLER, G. E. MATTHEWS, A. L. BIRCH, Assistant Examiners. 

1. IN A MASS RESOLVING DEVICE INCLUDING AN ION ACCELERATING ELECTRODE, MEANS CONNECTED TO THE ACCELERATING ELECTRODE FOR IMPRESSING UPON THE ACCELERATING ELECTRODE A PREDETERMINED HIGH-VOLTAGE DIRECT-CURRENT POTENTIAL DIFFERENT FROM GROUND POTENTIAL, AND FIRST AND SECOND SPACED ELECTRODE, THE SPARK-GAP ELECTRODE INCORPORATING A SAMPLE SPARK-GAP ELECTRODES DISPOSED ADJACENT THE ACCELERATING OF NON-GASEOUS MATERIAL TO BE IONIZED, APPARATUS FOR PRODUCING IONS OF THE SAMPLE ACCURATELY REPRESENTATIVE OF THE QUANTITATIVE COMPOSITION OF THE SAMPLE COMPRISING CIRCUIT MEANS COUPLED TO THE SPARK-GAP ELECTRODES FOR IMPRESSING A SELECTED SUBSTANTIALLY CONSTANT DIRECT-CURRENT POTENTIAL ACROSS THE SPACK-GAP ELECTRODES SUFFICIENT TO PRODUCE AN ARC ACROSS THE SPACE BETWEEN THE SPARK-GAP ELECTRODES AND FOR CAUSING THE FIRST SPARK-GAP ELECTRODE TO HAVE A POTENTIAL SUBSTANTIALLY EQUAL TO SAID PREDETERMINED POTENTIAL, THE CIRCUIT MEANS INCLUDING IMPEDANCE MEANS FOR LIMITING THE CURRENT WHICH FLOWS BETWEEN THE SPARK-GAP ELECTRODES DURING THE EXISTENCE OF AN ARC ACROSS SAID SPARK TO A VALUE INSUFFICIENT TO HEAT SAID SAMPLE TO A TEMPERATURE AT WHICH THE SAMPLE IONIZES SELECTIVELY IN RELATION TO THE VOLATILITY OF THE CONSTITUENTS THEREOF FOR ASSURING THAT THE SECOND SPARK-GAP ELECTRODE HAS A POTENTIAL DIFFERENT FROM GROUND POTENTIAL DURING THE EXISTENCE OF SAID ARC. 